THE SKELETAL SYSTEM 39 



A typical vertebra presents the following features: A body, which 

 is a cylindrical mass of bone upon which the other parts are built 

 up. The upper surface of the body is flattened and forms the 

 floor of the spinal canal. The anterior extremity presents a con- 

 vex head to articulate with the posterior concave surface of the 

 preceding vertebra. Between each two vertebrae there is a 

 cartilaginous disk which binds the bones firmly together. An 

 arch, which is built up on the upper surface of the body and com- 

 pletes the canal for the spinal cord. The processes, which are 

 of three varieties: (1) articular processes, which project forward 

 and backward from the arch; (2) transverse processes, usually 

 two in number, which project outward from the lower part of 

 the arch; (3) a spinous process, which is single and projects 

 upward from the arch. 



The cervical vertebrce form the skeleton of the neck. There are 

 seven bones in this group. The first two are modified so as to 

 allow the skull to move more freely. ' A typical member of this 

 group has an extremely long body with a large, strong arch and 

 spinal canal. The articular processes are well developed. The 

 transverse processes are wide and plate-like. Each spinous proc- 

 ess is small. The first cervical vertebra is called the atlas and 

 has no body, instead it is modified in the form of a ring. On 

 each side of the anterior edge of the ring is a deep articular cavity 

 for a condyle of the occipital bone of the skull. The posterior 

 edge articulates with the axis. The axis, or second cervical 

 vertebra, has a very long body. The anterior extremity presents 

 centrally a projection called the odontoid process. The posterior 

 extremity has the usual concave articular surface. The spine is 

 prominent, wide, and thick. 



The thoracic, or dorsal vertebras, are usually eighteen in number, 

 in the horse, sometimes nineteen, rarely seventeen. The body 

 is short and presents laterally two concave facets for articulation 

 with the heads of the ribs. The arch is small. The articular 

 processes are slightly developed; the transverse processes are 

 short and thick, and each has a facet for articulation with the 

 tubercle of a rib; the spinous process is long, and together they 

 determine the contour of the back and withers (see frontispiece). 



The lumbar vertebra; comprise the six bones in the region of 

 the loins. The bodies are longer than those of the dorsal region. 



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